Pumps for pumping molten metal can include a motor driven impeller on the end of a shaft inside an impeller chamber of an elongated base having an inlet and outlet from the impeller chamber. Upon rotation of the impeller, molten metal is drawn into the base into the impeller chamber and then travels to the outlet of the base. If the pump is a circulation or submerged discharge pump, the outlet of the base extends as a passageway to the outer surface of the base. Molten metal released from the pump circulates through a furnace or hearth, for example. If the pump is a transfer pump, the outlet can lead to a riser spaced apart from the shaft, which extends above the pump to a conduit which directs the molten metal to another location such as to a ladle or to a die casting machine. All of the components of the pump that are in the molten metal environment are typically made of refractory material such as graphite, ceramic, graphite with a ceramic covering or graphite impregnated with a refractory oxide.
Pumps of the type that include a base have been designed with a refractory shaft sleeve that extends between and is fastened to the motor support plate at its upper end and to the base at its lower end. The shaft rotates inside the sleeve. Gas has been added into the shaft sleeve as disclosed in U.S. Pat. No. 5,676,520, and displaced the molten metal inside. However, this can lead to cavitation of the pump as it can be difficult to control the gas.
A straining member has been used with such pumps having openings that prevent debris from entering the inlet (U.S. Pat. No. 6,533,535). In addition, shaft sleeves, including those of the 5,676,520 patent, have been formed with openings that permit entry of molten metal into the shaft sleeve and then into the base.
Apparatus exists for degassing in which a rotor is rotated in the molten metal while gas is added to the molten metal. Normally the shaft that carries the rotor extends between the motor and rotor and is uncovered. These devices usually do not include an enclosed submerged base in which the rotor rotates. The rotor rotates uncovered inside a vessel of molten metal. Gas can flow inside the shaft to the rotor. The gas can be argon or nitrogen and is used to remove hydrogen gas from the molten aluminum. The inventor's company, High Temperature Systems, Inc., sells such a degassing apparatus without a base, having a shaft sleeve below the motor and around the shaft, which extends downward to a location above the rotor.
The field of molten metal pumping continues to suffer from a longstanding problem of dross formation. Dross includes oxides of the molten metal, such as aluminum oxide in molten aluminum processing. The dross is periodically skimmed from a surface of the bath by a worker in a dangerous, labor intensive procedure. The dross formation increases due to agitation of the molten metal such as that occurring during pumping, and exposure of the metal to oxygen in the air. Remaining amounts of the dross can cause a reduction in the quality of the metal parts that are formed from the molten metal. Therefore, it has long been desired, but not yet satisfactorily achieved, to pump molten metal in a bath having reduced dross formation.